HostEntityUrlPropSource should be able to support absolute or relative URLs, URL options, and different link templates

Overview

✨ In a content template, how can I link to an entity? Active added a prop source to get the absolute, canonical URL of the host entity. Right now, it can only get the canonical URL, and it can only get it as an absolute URL. URL options (like query strings, for example) are not supported.

Proposed resolution

We might want to support some, or all, of those things. This issue is to decide what we'd like to support here, and implement it:

• Do we want to support more than just the canonical link template? (This would allow linking to, say, the edit form for an entity, or some other useful link template.)
• Do we want to allow the generated URL to be relative, instead of always absolute? I'm not entirely sure it matters, but it's worth considering.
• URL options, like query strings or document fragments -- do we want to support those? I think fragments might be quite useful in some situations (linking to anchors on a page) -- how would we do this?

User interface changes

TBD